Afforestation and stand development can significantly affect soil respiration and net ecosystem productivity (NEP). I studied 1) the effects of afforestation on NEP by comparing cropland previously planted to barley (on a barley-barley-alfalfa-alfalfa-alfalfa rotation) and that converted to a hybrid poplar (Populus deltoides × Populus × petrowskyana var. Walker) plantation and 2) the NEP along a chronosequence of stands aged 5-, 8-, 14-, and 16-year old in 2009 in central Alberta, Canada. Soil respiration and NEP decreased in the first two to three years after afforestation, while both generally increased with stand development. The ecosys model was used to simulate carbon dynamics in the plantations over a 20-year rotation under contrasting soil conditions. Soil conditions of the 14-year-old plantation accumulated the greatest amount of ecosystem carbon over the whole rotation. The research indicated that plantations could be a net carbon source in the first few years after afforestation and then became a net carbon sink, helping to mitigate net CO2 emissions for the remainder of the rotation.